An explosive formed penetrator (EFP), also known as an explosive formed projectile, is a self-forging warhead or a self-forging fragment from a special type of shaped charge designed to penetrate armor at standoff distances. The effect of explosive charge is to deform a metal plate into a slug or rod shape as well as accelerating it towards the targets. A conventionally-shaped charge generally has a conical metal liner that projects a hypervelocity jet of metal able to penetrate to great depths into steel armor. In travel over some distance, the jet breaks up along its length into particles that drift out of alignment, greatly diminishing its effectiveness at a distance. In contrast, an EFP has a liner face in the shape of a shallow dish. Thus, the force of the blast molds the liner into any of a number of shapes, depending on the shape of the plate and how the explosive is detonated. Some sophisticated EFP warheads have multiple detonators that can be fired in different arrangements causing different types of waveform in the explosive, resulting in a long-rod penetrator, an aerodynamic slug projectile, or multiple high-velocity fragments. A less sophisticated approach for changing the formation of an EFP is through the use of a wire-mesh in front of the liner where the mesh in place of the liner fragments into multiple penetrators.
The EFP are typically buried in the ground or hidden under debris to avoid detection. One example system for detecting EFP's is through the use of Millimeter Wave (MMW) radiometers that are passive radars measuring thermal radiations emitted from targets at MMW frequencies and represented by the brightness temperature. Millimeter Wave is the radiation band with wavelengths from about one millimeter to about 10 millimeters. As radio waves are considered low band frequency, millimeter waves are designated as very high frequency ranging from 30-300 GHz. Regardless of the frequency of the radiation, just like light, millimeter waves can be found throughout the environment. One problem with existing millimeter wave systems is that they have trouble differentiating an arbitrary wet dielectric surface from a metallic surface which can confuse the detection system when attempting to identify an EFP. One attempt at mitigating this problem is through the use of a Weiner filter. However, the Weiner filter is not capable of suppressing extraneous clutter surrounding the EFP image since such filters are only capable of suppressing noise through statistical approaches.